1. Field of the Invention
The present invention pertains to high aspect ratio performance coatings for biological microfluidics. The microfluidics may be with respect to fluid flow into openings on the surface of a substrate, with respect to flow of a fluid through an enclosed conduit within a substrate, or with respect to fluid flow within an enclosed conduit in general.
2. Brief Description of the Background Art
In the biological field, the surface characteristics of a substrate control the functioning of that substrate relative to fluids with which the substrate surface comes in contact. Since known living organisms rely heavily on the presence of water, the hydrophilicity or hydrophobicity of a given surface plays a major role in determining whether a fluid which is derived from a biological substrate or which contains a biological component will be compatible with the surface.
In the fields of biological testing and evaluation, medical devices for use in applications where contact with bio-fluids is required, and pharmaceuticals which are to contact a surface or to be delivered in encapsulated form for subsequent release into a bio-fluid environment (by way of example and not by way of limitation), the degree of hydrophilicity of a surface may be critical. The desired degree of hydrophilicity can be provided using a coating created by the method of the present invention.
In many applications where the wear on the coating is likely to occur due to mechanical contact or where fluid flow is to occur over the substrate surface on which the layer of coating is present, it is helpful to have the coating chemically bonded directly to the substrate surface via chemical reaction of active species which are present in the coating reactants/materials with active species on the underlying substrate surface. In addition, particular precursor materials may be selected which are known to provide particular functional moieties.
With respect to layers and coatings which are chemically bonded to the substrate surface, there are a number of areas of particular current interest. By way of example, and not by way of limitation, such coatings may be used for biotechnology applications, where the surface wetting properties and functionality of the coating are useful for analytical purposes, for controlling fluid flow and sorting of fluid components, and for altering the composition of components which come into contact with the surface, for example. Due to the nanometer size scale of some of applications for coatings exhibiting specialized functionality, a need has grown for improved methods of controlling the formation of the coating, including the formation of individual layers within a multi-layered coating. Historically, these types of coatings were deposited by contacting a substrate surface with a liquid phase. While this technique enables efficient coating deposition, it frequently results in limited film property control. In the case of coating a surface of a nanometer scale device, use of liquid phase processing limits device yield due to contamination and capillary forces. More recently, deposition of coatings from a vapor-phase has been used in an attempt to improve coating properties. However, the common vapor-phase deposition methods may not permit sufficient control of the molecular level reactions taking place during the deposition of surface bonding layers or during the deposition of functional coatings, when the deposited coating needs to function on a nanometer (nm) scale.
For purposes of illustrating methods of coating formation where vaporous and liquid precursors are used to deposit a coating on a substrate, applicants would like to mention the following publications and patents which relate to methods of coating formation, for purposes of illustration. Most of the background information provided is with respect to various chlorosilane-based precursors; however it is not intended that the present invention be limited to this class of precursor materials. In addition, applicants would like to make it clear that some of this Background Art is not prior art to the present invention. It is mentioned here because it is of interest to the general subject matter.
In an article by Barry Arkles entitled “Tailoring surfaces with silanes”, published in CHEMTECH, in December of 1977, pages 766-777, the author describes the use of organo silanes to form coatings which impart desired functional characteristics to an underlying oxide-containing surface. In particular, the organo silane is represented as RnSiX(4-n) where X is a hydrolyzable group, typically halogen, alkoxy, acyloxy, or amine. Following hydrolysis, a reactive silanol group is said to be formed which can condense with other silanol groups, for example, those on the surface of siliceous fillers, to form siloxane linkages. Stable condensation products are said to be formed with other oxides in addition to silicon oxide, such as oxides of aluminum, zirconium, tin, titanium, and nickel. The R group is said to be a nonhydrolyzable organic radical that may possess functionality that imparts desired characteristics. The article also discusses reactive tetra-substituted silanes which can be fully substituted by hydrolyzable groups and how the silicic acid which is formed from such substituted silanes readily forms polymers such as silica gel, quartz, or silicates by condensation of the silanol groups or reaction of silicate ions. Tetrachlorosilane is mentioned as being of commercial importance since it can be hydrolyzed in the vapor phase to form amorphous fumed silica.
The article by Dr. Arkles shows how a substrate with hydroxyl groups on its surface can be reacted with a condensation product of an organosilane to provide chemical bonding to the substrate surface. The reactions are generally discussed and, with the exception of the formation of amorphous fumed silica, the reactions are between a liquid precursor and a substrate having hydroxyl groups on its surface. A number of different applications and potential applications are discussed.
In an article entitled “Organized Monolayers by Adsorption. 1. Formation and Structure of Oleophobic Mixed Monolayers on Solid Surfaces”, published in the Journal of the American Chemical Society, Jan. 2, 1980, pp. 92-98, Jacob Sagiv discussed the possibility of producing oleophobic monolayers containing more than one component (mixed monolayers). The article is said to show that homogeneous mixed monolayers containing components which are very different in their properties and molecular shape may be easily formed on various solid polar substrates by adsorption from organic solutions. Irreversible adsorption is said to be achieved through covalent bonding of active silane molecules to the surface of the substrate.
In June of 1991, D. J. Ehrlich and J. Melngailis published an article entitled “Fast room-temperature growth of SiO2 films by molecular-layer dosing” in Applied Physics Letters 58 (23), pp. 2675-2677. The authors describe a molecular-layer dosing technique for room-temperature growth of α-SiO2 thin films, which growth is based on the reaction of H2O and SiCl4 adsorbates. The reaction is catalyzed by the hydrated SiO2 growth surface, and requires a specific surface phase of hydrogen-bonded water. Thicknesses of the films is said to be controlled to molecular-layer precision; alternatively, fast conformal growth to rates exceeding 100 nm/min is said to be achieved by slight depression of the substrate temperature below room temperature. Potential applications such as trench filling for integrated circuits and hermetic ultrathin layers for multilayer photoresists are mentioned. Excimer-laser-induced surface modification is said to permit projection-patterned selective-area growth on silicon.
An article entitled “Atomic Layer Growth of SiO2 on Si(100) Using The Sequential Deposition of SiCl4 and H2O” by Sneh et al. in Mat. Res. Soc. Symp. Proc. Vol 334, 1994, pp. 25-30, describes a study in which SiO2 thin films were said to be deposited on Si(100) with atomic layer control at 600° K. (≅327° C.) and at pressures in the range of 1 to 50 Torr using chemical vapor deposition (CVD).
In U.S. Pat. No. 5,372,851, issued to Ogawa et al. on Dec. 13, 1995, a method of manufacturing a chemically adsorbed film is described. In particular a chemically adsorbed film is said to be formed on any type of substrate in a short time by chemically adsorbing a chlorosilane based surface active-agent in a gas phase on the surface of a substrate having active hydrogen groups. The basic reaction by which a chlorosilane is attached to a surface with hydroxyl groups present on the surface is basically the same as described in other articles discussed above. In a preferred embodiment, a chlorosilane based adsorbent or an alkoxyl-silane based adsorbent is used as the silane-based surface adsorbent, where the silane-based adsorbent has a reactive silyl group at one end and a condensation reaction is initiated in the gas phase atmosphere. A dehydrochlorination reaction or a de-alcohol reaction is carried out as the condensation reaction. After the dehydrochlorination reaction, the unreacted chlorosilane-based adsorbent on the surface of the substrate is washed with a non-aqueous solution and then the adsorbed material is reacted with aqueous solution to form a monomolecular adsorbed film.
In an article entitled “SiO2 Chemical Vapor Deposition at Room Temperature Using SiCl4 and H2O with an NH3 Catalyst”, by J. W. Klaus and S. M. George in the Journal of the Electrochemical Society, 147 (7) 2658-2664, 2000, the authors describe the deposition of silicon dioxide films at room temperature using a catalyzed chemical vapor deposition reaction. The NH3 (ammonia) catalyst is said to lower the required temperature for SiO2 CVD from greater than 900° K. to about 313-333° K.
U.S. Pat. No. 5,328,768 to Goodwin, issued Jul. 12, 1994, discloses a method and article wherein a glass substrate is provided with a more durable non-wetting surface by treatment with a perfluoroalkyl alkyl silane and a fluorinated olefin telomer on a surface which comprises a silica primer layer. The silica primer layer is said to be preferably pyrolytically deposited, magnetron sputtered, or applied by a sol-gel condensation reaction (i.e. from alkyl silicates or chlorosilanes). A perfluoroalkyl alkyl silane combined with a fluorinated olefin telomer is said to produce a preferred surface treatment composition. The silane/olefin composition is employed as a solution, preferably in a fluorinated solvent. The solution is applied to a substrate surface by any conventional technique such as dipping, flowing, wiping, or spraying.
U.S. Patent Publication No. U.S. 2002/0065663 A1, published on May 30, 2002, and titled “Highly Durable Hydrophobic Coatings And Methods”, describes substrates which have a hydrophobic surface coating comprised of the reaction products of a chlorosilyl group containing compound and an alkylsilane. The substrate over which the coating is applied is preferably glass. In one embodiment, a silicon oxide anchor layer or hybrid organo-silicon oxide anchor layer is formed from a humidified reaction product of silicon tetrachloride or trichloromethylsilane vapors at atmospheric pressure. Application of the oxide anchor layer is, followed by the vapor-deposition of a chloroalkylsilane. The silicon oxide anchor layer is said to advantageously have a root mean square surface (RMS) roughness of less than about 6.0 nm, preferably less than about 5.0 nm and a low haze value of less than about 3.0 %. The RMS surface roughness of the silicon oxide layer is preferably said to be greater than about 4 nm, to improve adhesion. However, too great an RMS surface area is said to result in large surface peaks, widely spaced apart, which begins to diminish the desirable surface area for subsequent reaction with the chloroalkylsilane by vapor deposition. Too small an RMS surface is said to result in the surface being too smooth, that is to say an insufficient increase in the surface area/or insufficient depth of the surface peaks and valleys on the surface.
Simultaneous vapor deposition of silicon tetrachloride and dimethyldichlorosilane onto a glass substrate is said to result in a hydrophobic coating comprised of cross-linked polydimethylsiloxane which may then be capped with a fluoroalkylsilane (to provide hydrophobicity). The substrate is said to be glass or a silicon oxide anchor layer deposited on a surface prior to deposition of the cross-linked polydimethylsiloxane. The substrates are cleaned thoroughly and rinsed prior to being placed in the reaction chamber.
U.S. Patent Publication No. 2003/0180544 A1, published Sep. 25, 2003, and entitled “anti-Reflective Hydrophobic Coatings and Methods, describes substrates having anti-reflective hydrophobic surface coatings. The coatings are typically deposited on a glass substrate. A silicon oxide anchor layer is formed from a humidified reaction product of silicon tetrachloride, followed by the vapor deposition of a chloroalkylsilane. The thickness of the anchor layer and the overlayer are said to be such that the coating exhibits light reflectance of less than about 1.5%. The coatings are said to be comprised of the reaction products of a vapor-deposited chlorosilyl group containing compound and a vapor-deposited alkylsilane.
Other known related references pertaining to coatings deposited on a substrate surface from a vapor include the following, as examples and not by way of limitation. U.S. Pat. No. 5,576,247 to Yano et al., issued Nov. 19, 1996, entitled: “Thin layer forming method where hydrophobic molecular layers preventing a BPSG layer from absorbing moisture”. U.S. Pat. No. 5,602,671 of Hornbeck, issued Feb. 11, 1997, which describes low surface energy passivation layers for use in micromechanical devices. An article entitled “Vapor phase deposition of uniform and ultrathin silanes”, by Yuchun Wang et al., SPIE Vol. 3258-0277-786×(98) 20-28, in which the authors describe uniform, conformal, and ultrathin coatings needed on the surface of biomedical microdevices such as microfabricated silicon filters, in order to regulate hydrophilicity and to minimize unspecific protein adsorption. Jian Wang et al., in an article published in Thin Solid Films 327-329 (1998) 591-594, entitled: “Gold nanoparticulate film bound to silicon surface with self-assembled monolayers”, discuss a method for attaching gold nanoparticles to silicon surfaces with a self aligned monolayer (SAM) used for surface preparation”.
Patrick W. Hoffmann et al., in an article published by the American Chemical Society, Langmuir 1997, 13, 1877-1880, describe the surface coverage and molecular orientation of monomolecular thin organic films on a Ge/Si oxide substrate. A gas phase reactor was said to have been used to provide precise control of surface hydration and reaction temperatures during the deposition of monofunctional perfluorated alkylsilanes. Complete processing conditions are not provided, and there is no description of the apparatus which was used to apply the thin films. T. M. Mayer et al. describe a “Chemical vapor deposition of fluoroalkylsilane monolayer films for adhesion control in microelectromechanical systems” in J. Vac. Sci. Technol. B 18(5), September/October 2000. This article mentions the use of a remotely generated microwave plasma for cleaning a silicon substrate surface prior to film deposition, where the plasma source gas is either water vapor or oxygen.
U.S. Pat. No. 6,576,489 to Leung et al., issued Jun. 10, 2003 describes methods of forming microstructure devices. The methods include the use of vapor-phase alkylsilane-containing molecules to form a coating over a substrate surface. The alkylsilane-containing molecules are introduced into a reaction chamber containing the substrate by bubbling an anhydrous, inert gas through a liquid source of the alkylsilane-containing molecules, and transporting the molecules with the carrier gas into the reaction chamber. The formation of the coating is carried out on a substrate surface at a temperature ranging between about 15° C. and 100° C., at a pressure in the reaction chamber which is said to be below atmospheric pressure, and yet sufficiently high for a suitable amount of alkylsilane-containing molecules to be present for expeditious formation of the coating.
Some of the various methods useful in applying layers and coatings to a substrate have been described above. There are numerous other patents and publications which relate to the deposition of functional coatings on substrates, but which appear to us to be more distantly related to the present invention. However, upon reading these informative descriptions, it becomes readily apparent that control of coating deposition on a molecular level is not addressed in adequate detail in most instances. When this is discussed, the process is typically described in generalized terms like those mentioned directly above, which terms are not enabling to one skilled in the art, but merely suggest experimentation. To provide a monolayer or a few layers of a functional coating on a substrate surface which is functional or exhibits features on a nanometer scale, it is necessary to tailor the coating precisely. Without precise control of the deposition process, the coating may lack thickness uniformity and surface coverage, providing a rough surface. Or, the coating may vary in chemical composition across the surface of the substrate. Or, the coating may differ in structural composition across the surface of the substrate. Any one of these non-uniformities may result in functional discontinuities and defects on the coated substrate surface which are unacceptable for the intended application of the coated substrate.
U.S. patent application Ser. No. 10/759,857 of the present applicants describes processing apparatus which can provide specifically controlled, accurate delivery of precise quantities of reactants to the process chamber, as a means of improving control over a coating deposition process. The subject matter of the '857 application is hereby incorporated by reference in its entirety. The focus of the present application is the control of process conditions in the reaction chamber in a manner which, in combination with delivery of accurate quantities of reactive materials, provides a uniform, functional coating on a nanometer scale. The coating exhibits sufficient uniformity of thickness, chemical composition and structural composition over the substrate surface that such nanometer scale functionality is achieved.